Moire-pattern evaluation in which a transluminated planar grating is projected by an objective upon a reference grating so that the Moire-pattern is formed as the superposition of the image of the flat grating and the reference grating is described in J. Sci. Instrum., Vol. 42, P. 607 to 610, although this system is not used for testing optical quality of large area transparent vehicle windshields which are mass produced or produced in serial production systems in the context of a more or less automatic fabrication process. This known Moire pattern evaluation as described in this publication serves a purely scientific purpose with respect to detecting small irregularities of small transparent plates with an area of several square centimeters.
In this system, monochromatic light is transmitted as a bundle of parallel light pencils or rays with the aid of special optics onto the planar grating and transluminates the latter. These optics, the planar grating, the plate whose irregularities are to be detected, the objective and the reference grating have more or less the same size. The reference grating in particular is not significantly smaller than the planar grating or the plate whose irregularities are to be detected.
There is no teaching in the reference whatever as to how this system might be employed in the optical testing of large area panes and a simple upscaling of this earlier system for automotive windshield optical testing would lead to an extremely expensive apparatus because it would require optical devices of large diameter which are costly to fabricate, especially if they are to have the desired precision and resolution.
There is, however, a method known for the qualitative optical testing of automotive glass panes, especially automobile windshields (see German Open Application DE-OS 3600199) which uses a planar grating in the form of a transparency slide or diapositive. This grating is projected with the aid of a projector upon a projection surface on which the reference grating is provided. An objective in this case is provided in the projector. The windshield to be subjected to optical testing is in the projection path, i.e. in the path of the light rays between the objection of the projector and the projection surface.
The measurement signal is obtained in the form of a refractive power deviation in the presence of an optical defect in the vehicle pane which results from a distortion of the projective grating by comparison with the projected grating and the pattern obtained in the absence of the optical defect.
This system has the disadvantage that visual inspection is required and this, of course, requires appropriate personnel. A measurement value can be obtained only in the center of a Moire strip at which poor resolution results. The refractive power is determined from the inclination of the Moire lines. The image quality of the Moire-pattern is poor because of a poor contrast arising from the fact that sharply projected grating lines are not fully absorbed on the darker regions of the projection wall or surface. In the superposition of two clear lines on the projection surface, the backscattered light intensity is substantially smaller than in the case of a transmission. The socalled shadow image which results in addition to the Moire pattern is superimposed upon it and tends to obscure the Moire pattern so that in some cases at least the Moire pattern is simply not capable of practical evaluation.
The processing of the Moire pattern in this conventional system is effected by electrical image processing in a strip sequence scanning which excludes efficient parallel computation process from being employed. As a consequence, the process time per measurement is comparatively high.
The determination of the inclination of the Moire strips cannot be reliably effected by this earlier system and thus the resolution of the system can require improvement. As a consequence, problems are encountered if this earlier windshield optical testing method is to satisfy the requirements of German Industrial Standard DIN 52 305 or ECE 43 which must be satisfied by the present invention.